This invention relates to an electroluminescent (EL) lamp and, in particular, to an EL lamp displaying a graphics image only when lit.
An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer can include a phosphor powder or there may be a separate layer of phosphor powder adjacent a dielectric layer. The phosphor powder radiates light in the presence of a strong electric field, using very little current. The front electrode is typically a thin, transparent layer of indium tin oxide or indium oxide and the rear electrode is typically a polymer binder, e.g. polyvinylidene fluoride (PVDF), polyester, vinyl, or epoxy, containing conductive particles such as silver or carbon. The front electrode is applied to a polymer film such as polyester or polycarbonate to provide mechanical integrity and support for the other layers.
It is often desired to have an EL lamp produce a graphic image when illuminated, e.g. text, numerals, a corporate logo, or other symbol. A graphic image can be produced by patterning one or both electrodes of the EL lamp, by applying an opaque graphic to an EL lamp ("overprinting"), or by adding a pre-printed, transparent layer to a lamp. A problem with overprinting a lamp is that the graphic is visible even when the lamp is not lit. Many customers for EL lamps want a graphic visible only when the lamp is lit.
A graphic is also visible when the front electrode is patterned, although the graphic may not be obvious. A patterned electrode is more costly than a continuous electrode and the resulting lamp is yet more costly because one must separately power each lamp in a panel in order to have each lamp equally bright. Providing appropriate conductive runs increases the cost of the panel. Patterning the rear electrode typically produces a fuzzy image because the rear electrode is a screen printed conductive ink and is much thicker than the front electrode. The image is also somewhat fuzzy because the "image" of the rear electrode is diffused in the phosphor layer.
It is known in the art to provide displays with images that are concealed when one or more lamps are off. For example, U.S. Pat. 3,362,284 (Gay) discloses incandescent lamps illuminating a small sign in which a first message, written in phosphorescent ink, is hidden behind a screen on which a second message is written. The background of the first message is painted the same color as the phosphorescent ink. When the lamps are lit, the second message is visible. After the lamps are extinguished, the second message is invisible and the phosphorescent glow of the first message is visible temporarily through the screen.
U.S. Pat, 3,397,334 (Motson) discloses a black mask overprinted on an EL lan@having an Inconel front electrode that is deposited on a roughened glass surface. The front electrode is only 50% to 75% transmissive and appears to be black ("of virtually the same depth" as the mask). The mask defines indicia that are visible when the lamp is lit. Aside from the fact that the single color available, black, is unsuited to many applications, e.g. watch faces, only a mask can be used. The mask covers a substantial part of the front electrode, shielding it from light and making the electrode appear black. Further, as described in the patent, the roughened surface of the glass contributes to the apparent color of the front electrode.
In view of the foregoing, it is therefore an object of the invention to provide an EL lamp for displaying a graphic image only when lit.
Another object of the invention is to provide an overprinted EL lamp having a hidden graphic that is revealed only when the lamp is lit.
A further object of the invention is to provide a overprinted EL lamp in which a continuously visible graphic is combined with a graphic visible only when the lamp is lit.
Another object of the invention is to provide an EL lamp of any desired color having a hidden graphic that is revealed only when the lamp is lit.